This invention relates to an anti-lock control system for vehicle brakes.
When the brakes of a vehicle are applied, a braking force between the tire and the road surface is generated that is dependent upon various parameters including the road surface condition and the amount of slip between the tire and the road surface. For a given road surface, the brake force between the tire and the road surface increases with increasing slip values to a peak force occurring at a critical slip value. As the value of slip increases beyond the critical slip value, the braking force between the tire and the road surface decreases. Stable braking results when the slip value is equal to or less than the critical slip value. However, when the slip value becomes greater than the critical slip value, braking becomes unstable resulting in sudden wheel lockup thereby reducing vehicle stopping distance and a deterioration in the lateral stability of the vehicle.
Numerous wheel lock control systems have been proposed to prevent the wheels from locking during a braking maneuver. Typically, these systems control the applied brake pressure to prevent the wheels from locking by releasing the applied brake pressure when an incipient lockup condition is sensed (which may be represented by the wheel deceleration exceeding a critical value) and reapplying the brake pressure when the wheel has recovered from the incipient wheel lock condition (which may be represented by the wheel acceleration during wheel speed recovery while the brake pressure is released decreasing to a value representing the wheel approaching a hypothetical zero-slip wheel speed. This type of system results in rapid cycling of the brake pressure and the wheel slip value around the critical slip value to maintain a stable braking operation.